The US Postal Service has begun using a pretty neat online augmented reality (AR) tool. Developed by AKQA/DC, the Virtual Box Simulator is a marker-based AR applet that enables you to use a webcam to find a box size and shape that fits a shipment item.
Print out a marker on paper and point your webcam at it: a 3D virtual box appears. Their video example (click ‘See how it works‘ on the right-hand menu) shows the size of the box and transparency being altered so that when you place your shipment on top of the paper its easy to see which one best fits.

Compared to similar applications, even though it only shaves a minute or two off your time at the post office counter, its still the most useful I’ve seen.